1. Field of the Disclosure
The invention relates to a crystalline silicon ingot and a method of fabricating the same, and more particularly, to a crystalline silicon ingot containing small-sized silicon crystal grains at a bottom thereof and large-sized silicon crystal grains at a top thereof and a method of fabricating the same. The background of the disclosure can refer to the following references:
[1] K. Fujiwara, W. Pan, K. Sawada, M. Tokairin, N. Usami, Y. Nose, A. Nomura, T. Shishida, K. Nakajima, Directional growth method to obtain high quality polycrystalline silicon from its melt, Journal of Crystal Growth 2006; 292:282-285; and
[2] T. Y. Wang, S. L. Hsu, C. C. Fei, K. M. Yei, W. C. Hsu, C. W. Lan, Grain control using spot cooling in multi-crystalline silicon crystal growth, Journal of Crystal Growth 2009; 311:263-267.
2. Brief Description of the Related Art
Crystal growth by casting polycrystalline silicon in a laboratory grade can attain the growth of facet dendrite in a bottom of a crucible. For example, the above-mentioned reference [1] proposes crystal growth in a lateral direction densely spreads on a bottom of a crucible by local undercooling, and then post-shaped structures grow upwards. Large-sized silicon crystal grains thereof have low defect density and a better dual crystal structure, sigma 3. Accordingly, made from a silicon wafer sliced from the crystalline silicon ingot produced in accordance with reference [1], solar cells can achieve higher photo-electron conversion efficiency.
However, in the extent of a scale for an industry grade, it is relatively difficult to have facet dendrite densely spread on a bottom of a crucible by local undercooling. Industry-grade polycrystalline silicon cast, affected by the crucible and the uniformity of heating the entirety, is performed with the increase of variances of controlling the initial undercooling degree. It results in the fact that the polycrystalline silicon grows with large-sized crystal grains and without any better dual crystal structure in the bottom of the crucible, so as to become a portion with higher defect density. The defect density dramatically increases during crystal growth such that the crystalline silicon ingot has a poor crystal quality and subsequently-formed solar cells have lower photo-electron conversion efficiency.